Migration of CO
2
through storage reservoirs can be monitored using time lapse seismic
reflection surveys. At the Sleipner Field, injected CO
2
is distributed throughout nine layers within the
reservoir. These layers are too thin to be seismically resolvable by direct measurement of the separation
between reflections from the top and bottom of each layer. Here we develop and apply an inverse method
for measuring thick ness changes of the shallowest layer. Our approach combines differences in traveltime
down to a specific reflection together with amplitude measurements to determine layer thicknesses from
time lapse surveys. A series of synthetic forward models were used to test the robustness of our inverse
approach and to quantify uncertainties. In the absence of ambient noise, this approach can unambiguously
resolve layer thickness. If a realistic ambient noise distribution is included, layer thicknesses of 1–6 m are
accurately retrieved with an uncertainty of ±0.5 m. We used this approach to generate a thickness map
of the shallowest layer. The fidelity of this result was tested using measurements of layer thickness
determined from the 2010 broadband seismic survey. The calculated volume of CO
2
within the shallowest
layer increases at a rate that is quadratic in time, despite an approximately constant injection rate into the
base of the reser voir. This result is consistent with a diminished growth rate of the areal extent of underlying
layers. Finally, the relationship between caprock topography and layer thickness is explored and potential
migration pathways that charge this layer are identified